As one of semiconductor device wafers, there is an SOI (Silicon On Insulator) wafer having a silicon layer formed on a silicon oxide film that is an insulator film. This SOI wafer has characteristics such as a small parasitic capacitance, a high radiation resistant capability, and others since a silicon layer (which may be referred to as an SOI layer hereinafter) of a substrate surface layer portion that serves as a device fabrication region is electrically separated from the inside of the substrate by a buried oxide film layer (a BOX layer). Therefore, effects such as a high-speed/low-power consumption operation, prevention of a soft error, and others are expected, and the SOI wafer is expected as a substrate for a high-performance semiconductor device.
As a typical method for manufacturing this SOI wafer, there is a wafer bonding method or an SIMOX method.
The wafer bonding method is, e.g., a method for forming a thermal oxide film on a surface of at least one of two silicon single crystal wafers, then closely attaching the two wafers to each other through this formed thermal oxide film, performing a bonding heat treatment to enhance bonding force, and thereafter reducing a film thickness of one wafer (a wafer for forming an SOI layer (which will be referred to as a bond wafer hereinafter)) by mirror polishing or the like to manufacture an SOI wafer. Further, as a method for reducing the film thickness, there is, e.g., a method for grinding or polishing a bond wafer to a desired thickness or a method for implanting at least either hydrogen ions or rare gas ions into a bond wafer in advance to form an ion implanted layer and delaminating the bond wafer at the ion implanted layer after bonding, and a wafer bonding method adopting the latter method is generally called an ion implantation delamination method.
In general, the SIMOX method is a method for ion-implanting oxygen into a single-crystal silicon substrate, then performing a high-temperature heat treatment (an oxide film forming heat treatment), and reacting the implanted oxygen and silicon with each other to form a BOX layer, thereby manufacturing an SOI substrate.
Of the two typical techniques, since the wafer bonding method has superiority that a thickness of the SOI layer or the BOX layer to be fabricated can be freely set, this method can be applied to use in various devices.
In particular, the ion implantation delamination method as one of the wafer bonding methods has characteristics, i.e., excellent film thickness uniformity in addition to the superiority and can obtain stable device characteristics on an entire wafer surface. However, when a thickness of the SOI layer is as thick as several the ion implantation delamination method alone cannot cope with this thickness since a maximum acceleration voltage of an ion implanter is limited. As a method that can solve this problem, there is a method for performing epitaxial growth on a surface of a bonded wafer fabricated based on the ion implantation delamination method (Patent Document 1). Using this method enables freely setting a thickness of the SOI layer to several μm and also obtaining high uniformity of the SOI layer thickness that cannot be acquired in a bonded wafer based on a grinding/polishing method.
On the other hand, in the bonded SOI wafer, there may be required an SOI wafer in which an SOI layer having a low resistivity (0.1 Ωcm or less) is used as a seed layer and an epitaxial layer having a normal resistivity (approximately 1 to 20 Ωcm) is formed thereon due to requirements in a device configuration.
Meanwhile, it is known that, in the bonded SOI wafer, the SOI layer side warps in a convex shape due to its cross-sectional configuration. When this warpage enlarges, it results into a failure in a photolithography step in a device manufacturing process. Thus, to suppress this warpage of the bonded SOI wafer, Patent Document 2 and Patent Document 3 disclose that a warpage that forms the bonded surface side into a concave shape is previously formed to a base wafer before bonding.
Moreover, Patent Document 4 discloses that a warpage can be reduced by adjusting oxide film thicknesses on upper and lower surfaces (a bonding surface side and a back side) of a base wafer in a bonded SOI wafer fabricated by decreasing a film thickness based on polishing.
Even in case of fabricating the bonded SOI wafer based on the ion implantation delamination method, although the SOI layer side warps in the convex shape due to its cross-sectional configuration, since the SOI layer to be formed is a thin film that is 1 μm or less (several 100 nm or less in many cases) when the SOI layer is formed based on the ion implantation delamination method, the warpage can be sufficiently reduced by equally forming oxide films on the upper and lower surfaces (the bonding surface side and the back side) of the base wafer as described in Patent Document 4.